Ion beam processing systems are used in a variety of applications for modifying the properties of a substrate during the fabrication of thin film devices, such as semiconductor and data storage devices. In particular, one or more etching processes may be used to remove and shape layers of material on the substrate. One conventional etching procedure involves the use of a working gas that is ionized into a plasma state at low pressures (i.e., at pressures less than about 1 mTorr). Ions are extracted from the plasma and accelerated toward the substrate by ion optics for ion beam etching (“IBE”) of the layers of material.
As device critical dimensions shrink, the need for improved process uniformity without sacrificing beam directionality has driven the search for improved ion sources. IBE uniformity is directly related to the beam current density and the distributions of ions and energetic neutrals, wherein charged ions are converted to neutrals during beam transport via charge exchange ion-atom collisions. The integrated beam particle flux should be independent of the impact position on the substrate. The angular distributions of the charged and neutral beam particles at the substrate are directly related to the angular properties of the trajectories of the ions extracted from the plasma by the optics of the source. To optimize process uniformity, the incident particle trajectories should be approximately parallel across the surface of the substrate.
Conventional ion sources of more recent design utilize a helical or coil antenna wrapped about a discharge vessel to generate an inductively coupled plasma (“ICP”) using high frequency electromagnetic field energy, including, for example, radio-frequency (“rf”) electromagnetic energy. The antenna of the ion source, when carrying an oscillating high frequency current, induces a time-varying magnetic field inside the discharge vessel. In accordance with Faraday's law, the time-varying magnetic field induces a solenoidal, high frequency electric field that accelerates electrons in an azimuthal direction within the discharge vessel and sustains the ICP. In such manner, the RF-ICP source may generate relatively high plasma densities, such as on the order of 1011 cm−3. To provide the initial “seed” electrons required to ignite the RF-ICP source, the plasma may be ignited by imbibing electrons generated by an electron source in the process chamber during the start-up period. Alternatively, an igniter may be provided within the plasma generator. This igniter may be a pair of electrodes attached to a spark generator.
Because low pressure ICPs are diffusion dominated, the plasma density and, thus the radial plasma ion flux distribution at the ion optics plane, is invariably convex, i.e., largest at the center of the ion source and decreasing radially outwardly from the center of the ion source. This results in non-uniform ion current density distribution of broad ion beams generated by such conventional ion sources.
Typical broad beam ion sources utilize a multi-electrode accelerator system for forming and accelerating the ions into a beam. The electrodes in this system are flat or dished multi-aperture plates, typically called grids, perforated with an array of aligned apertures through which ion beamlets are extracted. Overlap of these beamlets results in a single broad ion beam. One known method of compensating for the effect on the plasma density profile of the plasma non-uniformities described above is to radially vary the transparency of the grids so as to decrease the beam current density in the center. However, this compensation method has several limitations. Variations in the transparency of the ion optics cannot compensate for variations in the plasma density profile for different ion source operating conditions (e.g., rf power, beam voltage and current, gas type and pressure), for any time dependence of these factors between system maintenance periods, or for variations in source and ion optics. The variations in source and ion optics may be either short and/or long term service condition changes in a given etch module because of the effect of mass and thermal loads, or module-to-module variations due to differences in ion source or grid construction. In some instances, a concave or convex beam ion density distribution may be desirable so as to compensate for variations in other aspects of processing of the substrates, including, for example, beam spreading during transport to the substrate, clamp effects at the periphery of the substrate, variations in the thickness of the material layer being etched, or variations in the width of the etch mask features.
Localized variations in the radial and/or azimuthal density distributions of the plasma typically limit the uniformity of the IBE process. The location and shape of these variations are dependent on the operating conditions. The transparency of the grid optics cannot be easily modified to compensate for this dependence on operating conditions.
Known conventional devices may further include features for tuning ion flux uniformity within a grid-based RF-inductively coupled ion source. Such features may use a re-entrant vessel, positioned at a predetermined distance from a screen grid of the grid-assembly. By decreasing the predetermined distance, the ion flux within the center of the ion source may be suppressed. Additionally, extensions may be included to fine tune radial variations in the ion flux distribution so as to flatten out any asymmetric peaks in the plasma distribution. Another known device employs a movable “plasma shaper” which extends from a re-entrant vessel and into the plasma discharge chamber to provide an extended range of tuning of the ion flux distribution.
Thus, there exists a need for ion sources with improved control of the ion flux distribution, particularly over spatial processing dimensions greater than 8 inches in diameter. More particularly, there exists a need for devices and methods that are amendable to controlling the ion beam current density distribution at the outer periphery of the ion flux density distribution. Furthermore, there exists a need for ion sources with improved range of adjustment of the beam profile and improved control of azimuthal variations in the ion flux distribution.